BUG/MEDIUM: peers: enforce check on incoming table key type

BUG/MEDIUM: peers: enforce check on incoming table key type

The key type received over the peers protocol is not checked for
validity and as a result can crash the process when passed through
peer_int_key_type[] in peer_treat_definemsg(). The risk remains
very low since only trusted peers may exchange tables, however it
represents a risk the day haproxy supports new key types, because
mixing old and new versions could then cause the old ones to crash.
Let's add the required check in peer_treat_definemsg().

It is also worth noting that in this function a few protocol identifiers
of type int read directly from a var_int via intdecode() and that some
protocol aliasing may occur (e.g. table_id, table_id_len etc). This is
not supposed to be a problem but it could hide implementation bugs and
cause interoperability issues once fixed, so these should be addressed
in a future commit that will not be marked for backporting.

Thanks to Haruto Kimura (Stella) for finding and reporting this bug.

This fix needs to be backported to all stable versions.